1. Field of the Invention
The present invention relates to the control of earth-orbiting vehicles such as communications satellites and more particularly to a method and means for reducing satellite pointing errors due to yaw attitude errors following station keeping maneuvers.
2. Problem to be Solved
Communications satellites, such as Intelsat-VII FM1, have experienced large yaw attitude errors following north/south station keeping (S/K) maneuvers. The cause of the errors has been shown to be propellant shift or motion in the spacecraft that produces an internal momentum exchange. The magnitude of the errors poses a problem with regard to degrading the pointing performance of the spacecraft, causing it to exceed pointing requirements. This, in turn, causes some degradation of the communication payload signals, with the Ka and Ku band payloads being the most strongly effected.
There are several courses of action which can be taken in response to the problems posed by these phenomena. The available options include, for example, just accepting the large attitude errors, that is, make no modifications to the firmware and take no actions on orbit. This option, for spacecraft such as those of the Intelsat type and size, would result in a yaw attitude error of as much as 0.60 degrees. Since the ADCS transition mode yaw pointing budget is 0.30 degrees, such an error would cause the spacecraft to exceed pointing requirements. Also, due to quarter orbit coupling, the attitude error eventually moves from the yaw axis to the roll axis over a period of 6 hours. As the error moves into the roll axis, it is absorbed by the momentum wheels and subsequently dumped using unloads. As a result, the yaw attitude error can be expected to exceed the pointing budget for 2-4 hours. The long term momentum management (LTMM) controller is unable to fire roll unloads to reduce the yaw attitude error sooner than this because the yaw angle estimator has a six hour time constant.
Another of the options involves allowing the propellant shift to occur before the transition to on orbit mode. This involves spending approximately 15 minutes in either station keeping (S/K) mode or in an `8 msec PWPF` mode, both of which require additional fuel. More particularly, with the former approach, following an S/K mode maneuver, the spacecraft remains in S/K mode for an additional 15 minutes after the north/south (N/S) maneuver. This approach requires no modifications to the firmware. Remaining in S/K mode will allow the propellant movement to occur while the yaw axis is still under thruster control. The thrusters will maintain the yaw pointing within the ADCS S/K mode pointing budget of 0.05 degrees. There are two disadvantages to this approach. First, it will cause the roll and pitch axes to experience S/K errors for an additional 15 minutes. These errors are much larger than the on orbit pointing budget of 0.007 degrees for roll and 0.01 degrees for pitch. Second, this approach has a fuel penalty of approximately 11 kg in communications spacecraft such as contemplated.
With the latter approach, i.e., `8 msec` pulse width pulse frequency modulator (PWPF) mode, the same basic operation of remaining in S/K mode would be exercised, but with a reduced fuel penalty. In this option, after an S/K mode maneuver has been completed, during which the PWPF modulator operates with 16 msec pulsewidths, the spacecraft is commanded (via ground commands) to switch to `8 msec PWPF` mode, wherein a mask is used to reduce the 16 msec pulsewidth length to 8 msec. The control loops produce a requested analog control torque that is input to the PWPF modulator which produces a pulse train approximating the analog input signal. Accordingly, the spacecraft is in this mode a few minutes after the N/S maneuver and remains in this mode for approximately 15 minutes before executing a transition to on orbit mode. In the `8 msec PWPF` mode, the pitch axis is under wheel control, while the roll and yaw axes remain under thruster control. The thrusters use an 8 msec pulse width, rather than the 16 msec pulse width used for S/K mode. Pitch unloads are allowed in order to maintain the wheel speeds within their operating range. This approach has the same disadvantages as the former approach, but on a smaller scale. The roll and pitch axes will still experience S/K errors for an additional 15 minutes. There is still a fuel penalty, estimated at about 3 kg, versus 11 kg for remaining in S/K mode. Other options in this regard are to add the `8 msec PWPF` mode to the ADCS firmware in the final ROM version prior to launch or after launch using the reprogramming features of the ADCS firmware. The after launch mode would reside in RAM and require the operator to follow a slightly more complicated procedure than the prior to launch option.
It will accordingly be seen that it is a problem to control momentum accumulation due to propellant shift following a North/South station keeping maneuver in a spacecraft such as a communications satellite, and, while various solutions exist or have been proposed, it is desirable to achieve a solution which is comparatively simple to execute and which minimizes fuel consumption.
3. Objects
It is accordingly an object of the present invention to provide a method and means for controlling momentum accumulation due to propellant shift following a North/South station keeping maneuver in a spacecraft such as a communications satellite.
It is another object of the present invention to provide such a method and means for controlling momentum accumulation which is comparatively simple to execute and which minimizes fuel consumption.
It is a further object of the invention to provide a method and means which modifies the LTMM, rather than the S/K control mode, so that the momentum management functions in a wheel controlled mode rather than a thruster controlled mode.